The present invention relates to a spindle motor capable of rotating at high speed, which employs hydrodynamic bearings as radial and thrust bearings. More particularly, the present invention relates to a spindle motor which is designed to rotate with minimal vibrations irrespective of the position of the motor when used and hence is suitable for a hard disk driver (hereinafter referred to as simply "HDD").
With the development of HDDs with a high storage capacity and low power consumption, demand has been made for improvements in the performance of spindle motors which are used to drive them so that these spindle motors are even more suitable therefor.
FIG. 4 is a partially sectioned elevational view of a conventional spindle motor for an HDD. The spindle motor has a shaft support cylinder 22 in the center of a base 21. A group of stator coils 23 are secured to the outer periphery of the shaft support cylinder 22. A rotary shaft 25 is rotatably supported by the inner periphery of the shaft support cylinder 22 with ball bearings 24 interposed therebetween. The rotary shaft 25 has a support member 27 secured to the upper end thereof, the support member 27 being arranged such that hard disks are fixedly mounted on the outer peripheral surface thereof. The support member 27 has a group of rotor magnets 28 secured to the inner peripheral surface in opposing relation to the group of stator coils 23.
In the above-described spindle motor employing ball bearings, the level of vibration of the spindle motor depends on the internal clearances of the ball bearings. The level of vibration in the radial direction is substantially equal to the radial internal clearance of the ball bearings. Similarly, the level of vibration in the thrust direction is substantially equal to the internal clearance of the ball bearings. Measures have been taken to reduce the vibration caused by these internal clearances. for example, by preloading the ball bearings. However, no satisfactory level of vibration has heretofore been obtained, i.e., it has been only possible to achieve 0.5 microns or so in terms of the non-repeated component of the runout in the radial direction. In addition, preloading of ball bearings results in an increase in the required torque of the motor and hence is counterproductive to a desirous lowering of the power consumption of the HDD. Accordingly, as long as ball bearings such as those described above are used, it is in principle virtually impossible to further reduce vibrations of the spindle motor.
Under these circumstances, spindle motors which employ hydrodynamic bearings to achieve a highly accurate rotating performance have been proposed.
FIG. 5 is a sectional view of a spindle motor which employs hydrodynamic bearings, applied for which a patent was for in advance of this application, as U.S. patent application Ser. No. 07/506,183, filed Apr. 9, 1990 in the name of Hisabe et al. and assigned to the assignee of the present application, now U.S. Pat. No. 4,998,033. A base 31 has a support shaft 32 positioned on the central portion thereof. An annular thrust bearing member 33 is secured to the base 31, and a cylindrical radial bearing member 34 is concentrically secured to the support shaft 32. A plurality of equally spaced stator coils 35 are secured to the support shaft 32 above the cylindrical radial bearing member 34.
A rotor 36 which serves as a hard disk supporting member has a cap-shaped configuration. The ceiling portion at the upper end of the rotor 36 is loosely fitted on the upper end portion of the support shaft 32. The rotor 36 has an annular bearing member 37 secured to the lower end portion thereof, the bearing member 37 having an L-shaped cross-sectional configuration. The lower end portion of the bearing member 37 faces the thrust bearing member 33 to form a thrust hydrodynamic bearing having spiral grooves. The inner peripheral surface of the bearing member 37 faces the radial bearing member 34 to form a radial hydrodynamic bearing having herringbone-shaped grooves. A plurality of equally spaced rotor magnets 38 are secured to the inner periphery of the rotor 36 in opposing relation to the stator coils 35.
As the stator coils 35 are sequentially supplied with an electric current, the rotor 36 having the rotor magnets 38 begins to rotate and consequently pneumatic pressure is generated between the upper surface of the thrust bearing member 33 and the lower surface of the bearing member 37, thus forming a thrust hydrodynamic bearing. Similarly, pneumatic pressure is generated between the outer peripheral surface of the radial bearing member 34 and the inner peripheral surface of the bearing member 37, thus forming a radial hydrodynamic bearing. Since the bearing member 37 is supported without being in solid contact with the associated members, the spindle motor is capable of smoothly rotating at high speed. Accordingly, this spindle motor is free from the problem of friction and vibration in contrast to the prior art that employs ball bearings.
However, the above-described spindle motor still suffers from the problem that, when it is operated in a horizontal position (i.e., in a direction in which the direction of gravity is perpendicular to the shaft of the motor), a moment in the radial direction is generated due to the gravity of the rotor, causing the axis of the rotor to be inclined with respect to the support shaft, which results in an increase in the imbalance of radial magnetic force acting between the rotor magnets and the stator coils, and in this state the rotor is brought into local contact with the bearing.
In addition to the problem stated above, when the conventional spindle motors that employ hydrodynamic bearings are used in a horizontal position, the following problems are experienced:
(1) The level of vibration during rotation is large. PA0 (2) When two discrete bearings are used, it is difficult to align them concentrically at the time of assembly. In addition, since the clearance between a movable piece and a fixed piece of a radial bearing is of the order of microns, it is difficult to align them concentrically during the manufacturing process. In addition, since the thrust collar of a thrust bearing is produced so that the parallelism is within several microns, it is necessary to hold down the parallelism to about 1 micron when it is assembled, which is very difficult. PA0 (3) In a radial gap type spindle motor such as that shown in FIG. 5, a moment is generated due to the imbalance of radial magnetic force acting between the rotor magnet group and the stator coil group, causing the axis of the rotor to be inclined with respect to the support shaft, which results in an increase in the starting torque because of local contact of the dynamic pressure surfaces. In rotation, unstable radial magnetic force, which is added to the dynamic pressure, causes whirling of the shaft and therefore makes it impossible to obtain a satisfactory operating condition.
In view of the above-described circumstances, it is an object of the present invention to provide a spindle motor which employs hydrodynamic bearings to improve a high-speed rotating performance and minimize vibration irrespective of the position of the motor when used and which is therefore suitable for a high-recording capacity HDD.